1. Field of the Invention
This invention relates to a scan format conversion apparatus for reproducing television signal digitally on a television such as a large display unit or a plane display unit.
2. Description of Related Art
FIG. 1 schematically presents a simplified block diagram of a conventional scan format conversion apparatus which is made available for displaying an NTSC-format television signal (hereinafter merely called NTSC signal) on those plane display units using different scan formats. The reference numeral 1 shown in FIG. 1 designates an input terminal for receiving an NTSC signal. The reference numeral 2 designates an IDTV decoder consisting of a circuit which separates the luminance signal and the chrominance signal from the NTSC signal, demodulates the chrominance signal, and then converts the interlaced-scanning signal into a noninterlaced-scanning signal. The IDTV decoder 2 outputs a noninterlaced-scanning synchronizing signal to a clock/control pulse generating circuit 3, and also outputs noninterlaced-scanning Red(R.) Green(G.) and Blue(B.) signals to respective A/D converters 4, 5 and 6. Synchronous with the inputted synchronizing signal, the clock/control pulse generating circuit 3 generates clock and control pulses for delivery to the A/D converters 4, 5, 6, and to a memory controller 7.
The A/D converters 4, 5, 6 convert the inputted analog signals into respective digital R.G.B. signals, and then output the digitized R.G.B. signals to corresponding image memories 8, 9, 10. These image memories 8 through 10 write and store digitized signals outputted from respective A/D converters 4, 5, 6 every frame period, and then respectively output needed data signals to an R-signal output terminal 11, a G-signal output terminal 12, and a B-signal output terminal 13. In order to properly control data writing and reading operations of the image memories 8, 9, 10, the memory controller 7 outputs control signals to those image memories 8, 9, 10.
Next, the functional operation of the conventional scan format conversion apparatus cited above will be described.
Initially, an analog NTSC TV signal is delivered to the IDTV decoder 2 via the input terminal 1. Next, the IDTV decoder 2 sequentially converts the analog NTSC signal scanning lines into separate luminance and chrominance signals. The IDTV decode 2 then outputs the noninterlaced-scanning RGB signals representing 525 scan lines per frame at a frame frequency of 59.94 Hz. The IDTV decode 2 also outputs a corresponding synchronizing signal. Next, the clock/control pulse generating circuit 3 outputs clock and control pulses synchronized with the inputted synchronizing signal, to the A/D converters 4, 5, 6 and the memory controller 7.
An apparatus (hereinafter called a display unit) capable of displaying signals that are outputted from the R-signal output terminal 11, the G-signal output terminal 12, and the B-signal output terminal 13 must fully satisfy the aspect ratio prescribed for the NTSC signal. Therefore, the display unit has 4n.times.3n pixels (where n designates a positive integer). Since the NTSC signal contains 83% of the effective horizontal scanning rate, and assuming that the horizontal scanning frequency of the RGB signal sequentially converted by the IDTV decoder 2 is "fH", then, the value of the clock pulse "fs" re-sampled by the A/D converters 4, 5, 6 can be computed by applying the equation (1) shown below. EQU fs=4n.times.(100/83).times.fH (1)
The analog R-signal, G-signal, and B-signal outputted from the IDTV decoder 2 are respectively converted into digital signals by the respective A/D converters 4, 5, 6 at the rate "fs" computed from equation (1). These digitized signals outputted from those A/D converters 4, 5, 6 are respectively written in the corresponding image memories 8, 9, 10 every frame period. The memory controller 7 properly controls the operation for writing these digitized signals into those image memories 8, 9, 10.
The NTSC signal contains 490 effective scanning lines per frame. On the other hand, each display unit incorporates 3n number of pixels in the vertical direction. Consequently, the effective scanning lines should be converted from 490 scanning lines into 3n scanning lines. Conversion of the scanning lines is executed by reading data from those image memories 8, 9, 10. The relationship between the data DM(i) of those data written in the image memories 8, 9, 10 every frame period, corresponding to the i-th scanning lines (where i=1, 2, . . . , 490), and the data DO(k) corresponding to the k-th scanning line of the display unit (where k=1, 2, . . . , 3n) is ruled by the equation (2) shown below, where a linear conversion of the scanning line is executed. EQU DO(k)=DM(1+INT]489.times.(k-1)/(3n-1)]) (2)
where INT bracket [ ] designates the maximum integer that does not exceed the number shown in the above bracket [ ].
According to the above equation (2), the memory controller 7 properly controls the operation for reading data from the image memories 8, 9, 10. As a result, the scan-line converted RGB signals are delivered to the R-signal output terminal 11, the G-signal output terminal 12, and the B-signal output terminal 13 of the corresponding image memories 8, 9, 10.
Next, examples of the scan-line conversion based on the above equation (2) are respectively shown below by referring to the case in which (a)n=50 and (b)n=400. The case in which (a)n=50
Since the display unit contains 3n=150 of pixels in the vertical direction, the data DO(1) through DO(150) corresponding to the scanning lines of the display unit are computed in accordance with the above equation (2). ##EQU1##
The data DO(1) through DO(150) are respectively read by reading those data corresponding to the 490 scanning lines written in the image memories 8, 9, 10 at intervals of 3 or 4 scanning lines apart. The case in which (b)n=400
Since the display unit contains 3n=1200 pixels in the vertical direction, the data DO(1) through DO(1200) corresponding to the scanning lines of the display unit are computed in accordance with the above equation (2). ##EQU2##
The data DO(1) through DO(1200) are respectively read by reading the data corresponding to 490 scanning lines written in the image memories 8, 9, 10. It can be seen that, in this case, each of the 490 scan lines located in the image memories 8, 9, 10 is repeatedly used for several display unit scan lines.
Actually, any conventional scan format conversion apparatus incorporates the operating system described above. Consequently, the number of pixels in the vertical direction (i.e., the the number of scanning line) of all the signals delivered to the image memories 8, 9, 10 remains constant. When reproducing an image on a display unit that has more vertical pixels than the number of vertical pixels contained in the image signal, any of the conventional scan format conversion apparatuses needs to read the identical pixels from those image memories 8, 9, 10 twice. On the other hand, when reproducing an image on a display unit that has fewer vertical pixels the number of vertical pixels contained in the image signal, the conventional apparatus needs to read pixels at intervals of several pixels. Consequently a resulting problem is that oblique lines of an image cannot continuously be displayed.